Image forming apparatus with selective application of light source

ABSTRACT

An image forming apparatus includes: a light source; a light-source driving unit; a photosensitive member including an image forming region; a determining unit determining a level of a rotation speed of the photosensitive member; a scanner unit exposing the photosensitive member by deflecting the light beam from the light source by a deflecting unit; a sensor receiving the light beam and outputs a detection signal; and a control unit controlling the image forming apparatus based on the detection signal output from the sensor. When the determining unit determines that the rotation speed is low, the light-source driving unit is configured to drive the light source to emit the light beam when a deflecting direction of the light beam is oriented toward the sensor and not to drive the light source to emit the light beam when the deflecting direction of the light beam is oriented toward the image forming region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Japanese Patent Application No.2009-048304 filed on Mar. 2, 2009, the entire subject matter of which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an image forming apparatus.

BACKGROUND

There has been proposed a known image forming apparatuses that scans arotating photosensitive member with a laser light beam emitted from alaser diode. The known apparatus supplies the laser diode with a firstbias current set to a current value less than a light-emission thresholdvalue of the laser diode, regardless of whether the laser diode iscaused to emit light, in order to improve the responsiveness of thelaser diode.

SUMMARY

Illustrative aspects of the present invention provide an image formingapparatus that controls the image forming apparatus based on a lightbeam while inhibiting deterioration of a photosensitive member when arotation speed of the photosensitive member is low.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an image forming apparatusaccording to an exemplary embodiment of the invention;

FIG. 2 is a block diagram s of the image forming apparatus;

FIG. 3 is a schematic diagram of a scanner unit of the image formingapparatus;

FIG. 4 is a timing chart for explaining rotation control of aphotosensitive member of the image forming apparatus according to afirst exemplary embodiment;

FIG. 5 is a timing chart showing timings at which various types ofsignals and currents are output;

FIG. 6 is a timing chart according to a second exemplary embodiment ofthe present invention;

FIG. 7 is a timing chart according to a third exemplary embodiment ofthe present invention; and

FIG. 8 is a timing chart according to a fourth exemplary embodiment ofthe present invention.

DETAILED DESCRIPTION

<General Overview>

Incidentally, the photosensitive member is not constantly rotating, andmay temporarily stop rotating from time to time.

According to the known image forming apparatus, when the rotation of thephotosensitive member is stopped, even if the current value of the firstbias current is less than the light-emission threshold value, forexample, the same location of the photosensitive member is intensivelyirradiated with light output from the laser diode (light emitted whenthe current value is less than the light-emission threshold valueresults in light having an uneven wavelength rather than a laser lightbeam). This may accelerate the deterioration of the photosensitivemember.

Therefore, illustrative aspects of the present invention provide animage forming apparatus that controls the image forming apparatus basedon a light beam while inhibiting deterioration of a photosensitivemember when a rotation speed of the photosensitive member is low.

According to a first illustrative aspect of the present invention, thereis provided an image forming apparatus comprising: a light source thatemits a light beam; a light-source driving unit that drives the lightsource; a photosensitive member that comprises an image forming region;a determining unit that determines a level of a rotation speed of thephotosensitive member; a scanner unit that exposes the photosensitivemember by deflecting the light beam emitted from the light source by adeflecting unit; a sensor that receives the light beam and outputs adetection signal; and a control unit that controls the image formingapparatus based on the detection signal output from the sensor, whereinwhen the determining unit determines that the rotation speed is low, thelight-source driving unit is configured: to drive the light source toemit the light beam when a deflecting direction of the light beam isoriented toward the sensor; and not to drive the light source to emitthe light beam when the deflecting direction of the light beam isoriented toward the image forming region.

According thereto, even when it is determined that the rotation speed ofa photosensitive member is low, a light beam is emitted when adeflecting direction of the light beam is oriented toward a sensor.Thus, even when it is determined that the rotation speed of thephotosensitive member is low, it is possible to control an image formingapparatus based on the light beam.

Moreover, when it is determined that the rotation speed of thephotosensitive member is low, the light beam is not emitted when thedeflecting direction of the light beam is oriented toward an imageforming region. Thus, it is possible to inhibit the deterioration of thephotosensitive member, specifically, the deterioration of the imageforming region, caused due to the fact that the light beam irradiatedintensively the same location of the image forming region.

Therefore, according to this invention, when the rotation speed of thephotosensitive member is low, it is possible to control the imageforming apparatus based on the light beam while inhibiting thedeterioration of the photosensitive member.

According to a second illustrative aspect of the invention, in the imageforming apparatus, wherein an intensity of the light beam emitted fromthe light source varies according to a current supplied to the lightsource, wherein the light-source driving unit supplies: a first currentthat drives the light source to emit a light beam having a firstintensity for exposing the photosensitive member with an intensity ofequal to or more than a predetermined exposure amount; and a secondcurrent that drives the light source to emit a light having a secondintensity for exposing the photosensitive member with an intensity ofless than the predetermined exposure amount, and wherein when thedeflecting direction of the light beam is oriented toward the sensor,the light-source driving unit supplies the first current to the lightsource.

According thereto, when the deflecting direction of the light beam isoriented toward a sensor, the high-intensity light beam is caused toemit light. As a result, the accuracy for detecting the light beam isimproved.

According to a third illustrative aspect of the invention, in the imageforming apparatus, wherein when the determining unit determines that therotation speed of the photosensitive member is high, the light-sourcedriving unit constantly supplies the second current as a bias current tothe light-source driving unit.

According thereto, when it is determined that the rotation speed of thephotosensitive member is high, a light source is constantly suppliedwith a bias current. As a result, it is possible to improve theresponsiveness of the light source. Moreover, this bias current is a lowcurrent with which to emit low-intensity light that is obtained when aphotosensitive member is exposed with an intensity of less than apredetermined exposure amount. Thus, it is possible to inhibitdeterioration of the photosensitive member, caused when the bias currentis constantly supplied.

According to a fourth illustrative aspect of the invention, in the imageforming apparatus, wherein when the determining unit determines that therotation speed of the photosensitive member is low, the light-sourcedriving unit stops supplying the bias current to the light source andmakes a timing at which the light source is supplied with the firstcurrent is faster than a timing at which the first current is suppliedwhen the rotation speed of the photosensitive member is high.

When it is determined that the rotation speed of the photosensitivemember is low, if the bias current is constantly supplied, thedeterioration of the photosensitive member may be accelerated due to thefact that the same location is intensively irradiated with the lightbeam even in the case of a low-intensity light beam, for example.

According to the fourth illustrative aspect of the invention, when it isdetermined that the rotation speed of the photosensitive member is low,the bias current is not supplied. As a result, it becomes possible toinhibit the deterioration of the photosensitive member.

However, since no bias current is supplied, if the light beam is emittedwhen the deflecting direction of the light beam is oriented toward asensor, the responsiveness of the light source may be degraded.Therefore, with this invention, a timing at which the light source issupplied with a high current is faster than a timing at which the highcurrent is supplied when it is determined that the rotation speed of thephotosensitive member is high. This enables a decrease in degradation ofthe responsiveness of the light source.

According to a fifth illustrative aspect of the invention, in the imageforming apparatus, wherein when the determining unit determines that therotation speed of the photosensitive member is low, the light-sourcedriving unit stops supplying the bias current to the light source andadjusts a value of the first current supplied to the light source tosubstantially the same value as the first current supplied to the lightsource when the rotation speed of the photosensitive member isdetermined to be high.

Further, according to a sixth illustrative aspect of the invention, inthe image forming apparatus, wherein when the determining unitdetermines that the rotation speed of the photosensitive member is low,the light-source driving unit stops supplying the bias current to thelight source and adjusts a value of the first current supplied to thelight source to a value smaller than that of the first current suppliedto the light source when the rotation speed of the photosensitive memberis determined to be high.

According thereto, it is possible to ensure that the current valuesupplied to the light source does not exceed a rating.

According to a seventh illustrative aspect of the invention, in theimage forming apparatus, wherein when the determining unit determinesthat the rotation speed of the photosensitive member is low, thelight-source driving unit is configured: not to supply the bias currentwhen the deflecting direction of the light beam is oriented toward theimage forming region; and to supply the bias current when the deflectingdirection of the light beam is oriented toward the sensor.

According thereto, it is possible to make it difficult for a problem toeasily arise due to the fact that there is no bias current.

According to an eighth illustrative aspect of the invention, in theimage forming apparatus, wherein the light-source driving unit is keptin an operable state even when the rotation speed of the photosensitivemember is decreased.

According thereto, it is possible to reduce a decrease in responsivenessof the light-source driving unit.

According to a ninth illustrative aspect of the invention, the imageforming apparatus further comprises: a speed detecting unit that detectsthe rotation speed of the photosensitive member, wherein the determiningunit determines the level of speed by comparing a speed detected by thespeed detecting unit and a predetermined speed.

According thereto, when a speed detected by the speed detecting unit anda predetermined speed are compared, it becomes possible to determine thelevel of the rotation speed of the photosensitive member.

According to a tenth illustrative aspect of the invention, the imageforming apparatus further comprises: a photosensitive-member drivingunit that rotationally drives the photosensitive member; and a conveyingunit, which conveys a recorded medium one at a time, and which, after animage is formed on a first surface of the recorded medium, reverses andconveys the recorded medium by using power of the photosensitive-memberdriving unit in order to form an image on a second surface of therecorded medium, wherein when the conveying unit reverses the recordedmedium, the power of the photosensitive-member driving unit is appliedto the conveying unit so as to stop a rotation of the photosensitivemember.

According thereto, even when the rotation of the photosensitive memberis stopped as a result of power of photosensitive-member driving unitbeing used when conveying unit reverses a recorded medium, it ispossible to control the image forming apparatus based on the light beamwhile inhibiting the deterioration of the photosensitive member.

According to the illustrative aspects of the present invention, when therotation speed of the photosensitive member is low, it is possible tocontrol an image forming apparatus based on a light beam whileinhibiting the deterioration of a photosensitive member.

<Exemplary Embodiments>

Exemplary embodiments of the invention will now be described withreference to the drawings.

(First Exemplary Embodiment)

A first exemplary embodiment of the present invention will be describedwith reference to FIG. 1 to FIG. 5.

(1) Image Forming Apparatus

FIG. 1 is a cross-sectional side view of main parts of an image formingapparatus 1. The image forming apparatus 1 includes such as a main bodyframe 11, a feeder unit 12, a conveying mechanism 13, an image formingunit 14, and a conveying motor M. Incidentally, a laser printer is oneexample of the image forming apparatus 1.

The feeder unit 12 includes a sheet feeding tray 16 on which recordedmedia such as a print sheet 15 are loaded, a press plate 17, and a sheetfeeding roller 18. The press plate 17 is capable of rotating around itsrear end part, and the print sheet 15 on the press plate 17 is pressedtoward the sheet feeding roller 18. When the sheet feeding roller 18 isrotated, the print sheet 15 is sent out to a conveying path one by one.

The conveying mechanism 13 (one example of a conveying unit) includes: apair of registration rollers 19; a pair of sheet discharge rollers 20; adrive roller 21; a driven roller 22; a belt 23 rolled around the driveroller 21 and the driven roller 22; two driven rollers 24 and 25 placedwithin the belt 23; and four press rollers 26, 27, 28, and 29 placed, bysandwiching the belt 23, to correspond to the drive roller 21 and thethree driven rollers 22, 24, and 25. The print sheet 15 sent out fromthe feeder unit 12 onto the conveying path is registered by theregistration rollers 19, and then, forwarded to a transferred positionX. The transferred position X is a position at which a toner image on aphotosensitive drum 30 (one example of a photosensitive member) istransferred to the print sheet 15, and is a position at which thephotosensitive drum 30 and a transfer roller 31 come into contact.

Moreover, the conveying mechanism 13 is capable of conveying the printsheet 15 so that after an image is formed on one surface (top surface)of the print sheet 15, the print sheet 15 is automatically reversed andthe image is then formed on the other surface (bottom surface) thereof.Specifically, when the rear end of the print sheet 15 is sandwiched bythe sheet discharge rollers 20, a flapper 32 is rotated clockwise asindicated by the broken line in FIG. 1, and then, a switchback path 33is released. Thereafter, the sheet discharge rollers 20 are reverselyrotated. Thereby, the print sheet 15 is sent out to the switchback path33 from a rear end side. The print sheet 15 sent out to the switchbackpath 33 is again sent out, with its bottom side up, to the conveyingpath from the rear end side. In this way, the image can be formed on thebottom surface.

An image forming unit 14 includes a scanner unit 34, a process cartridge35, and a fixing part 36.

The scanner unit 34 includes a polygon mirror 37 (one example of adeflecting unit), and a laser diode 47 (one example of a light source,see FIG. 3). A light beam (dashed line in FIG. 3) caused to emit lightfrom the laser diode 47 (hereinafter, referred to as “LD”) is deflectedby the polygon mirror 37 and irradiates onto the top surface of aphotosensitive drum 30. The scanner unit 34 will be described in detaillater.

The process cartridge 35 includes a developing roller 38, aphotosensitive drum 30, and a scorotron-type charger 39. The charger 39uniformly charges the top surface of the photosensitive drum 30 to apositive polarity. The top surface of the photosensitive drum 30 chargedto the positive polarity is exposed by the light beam caused to emitlight from the scanner unit 34 so that an electrostatic latent image isformed. Subsequently, toner carried on the top surface of the developingroller 38 is supplied to the electrostatic latent image formed on thephotosensitive drum 30, and then, the resultant toner is developed.

The fixing part 36 causes the toner to be thermally fixed onto the printsheet 15 while the print sheet 15 passes between a heating roller 40 anda press roller 41. The print sheet 15 that has been thermally fixed isdischarged onto a sheet discharge tray 43 via a sheet discharge path 42.

A conveying motor M (one example of a photosensitive-member drivingunit) is commonly used as a driving source for rotationally driving thephotosensitive drum 30 and the developing roller 38 and as a drivingsource for driving the conveying mechanism 13. The conveying mechanism13 reverses the print sheet 15 by using the driving force of theconveying motor M. At this time, the conveying motor M is reverselyrotated. When the photosensitive drum 30 and the developing roller 38are also reversely rotated if the conveying motor M is reverselyrotated, various types of problems arise. To avoid this, it isconfigured such that the photosensitive drum 30 and the conveying motorM (or the developing roller 38 and the conveying motor M) are coupled sothat a rotationally driving force is transmitted in a single directiononly via a one-way clutch, etc., and when the print sheet 15 isreversed, the rotation of the photosensitive drum 30 is stopped.

(2) Electrical Configuration of Image Forming Apparatus

Referring to FIG. 2, an electrical configuration of the image formingapparatus 1 will be described.

The image forming apparatus 1 includes a CPU 60, a ROM 61, a RAM 62, anEEPROM 63, a BD light-receiving sensor 51, an LD driving circuit 48, afeeder unit 12, an image forming unit 14, a display unit 64, and anoperating unit 65. In FIG. 2, the BD light-receiving sensor 51 and theLD driving circuit 48 are shown separately from the image forming unit14; however, these components configure part of the image forming unit14.

The CPU 60 (one example of a light-source driving unit, a determiningunit, a speed detecting unit, and a control unit) executes various typesof programs stored in the ROM 61 thereby to control each part of theimage forming apparatus 1. In the ROM 61, various types of programsexecuted by the CPU 60 and tables referred to when the CPU 60 executesvarious types of processes are stored. The RAM 62 is used as a primarystorage device for the CPU 60 to execute various types of processes. TheEEPROM 63 is a nonvolatile memory capable of storing information evenwhen the power is stopped, and stores various types of settinginformation, etc.

The display unit 64 is configured by various types of lamps, a liquidcrystal panel, etc. The operating unit 65 is configured by an inputpanel, etc., and a user is capable of giving a print command, forexample, by operating the operating unit 65 with reference to thedisplay unit 64.

The BD light-receiving sensor 51 and the LD driving circuit 48 will bedescribed in detail later.

In addition, the image forming apparatus 1 includes a network interface(not shown) for connecting to an external device are arranged. The useris also capable of giving a print command from the external device viathe network interface.

(3) Scanner Unit

FIG. 3 is a schematic diagram showing the scanner unit 34 from adifferent angle from FIG. 1. The scanner unit 34 includes: an LD 47emitting a light beam; an LD driving circuit 48 for controlling the LD47; a first lens unit 49 configured by a collimator lens, a cylindricallens, etc.; the polygon mirror 37; a second lens unit 50 configured byan fθ lens, a cylindrical lens, etc.; a BD light-receiving sensor 51, apolygon motor (not shown) for driving the polygon mirror; and apolygon-motor driving circuit (one example of a control unit) (notshown) for controlling the polygon motor.

The LD driving circuit 48 (one example of a light-source driving unit)includes: a switching current circuit for supplying the LD 47 with aswitching current (one example of a high current) with which to cause ahigh-intensity light beam (laser light beam), which is obtained when thephotosensitive drum 30 is exposed with light having an intensity ofequal to or more than a predetermined exposure amount, to emit light;and a bias current circuit for supplying the LD 47 with a bias current(one example of a low current) with which to emit low-intensity light(light having an uneven wavelength rather than a laser light beam) thatis obtained when the photosensitive drum 30 is exposed with light havingan intensity of less than the predetermined exposure amount.

The “predetermined exposure amount” is an exposure amount that acts as aboundary in determining whether the photosensitive drum 30 is exposed tothe extent that the quality of the image is affected when thephotosensitive drum 30 rotating at a speed equal to or faster than apredetermined speed is exposed. The level of the exposure amount of the“predetermined exposure amount” is appropriately determined by adesigner, et al., through experimentation, etc. The “high-intensitylight beam exposed with an intensity of equal to or more than apredetermined exposure amount” is a light beam having an intensity ofequal to or more than the above-described exposure amount. The“low-intensity light exposed with an intensity of less than thepredetermined exposure amount” is light having an intensity of less thanthe above-described exposure amount.

The LD driving circuit 48 supplies the LD 47 with an switching currentwhen a Video signal is output from the CPU 60, and supplies the LD 47with a bias current when a bias signal is output.

The light beam caused to emit light from the LD 47 irradiates onto thepolygon mirror 37 via the first lens unit 49. The polygon mirror 37 isrotated at high speed by the polygon motor. The irradiated light beam isdeflected by the polygon mirror 37, and irradiates onto thephotosensitive drum 30 via the second lens unit 50.

The polygon mirror 37 is configured by six mirror surfaces, andcyclically deflects the light beam. In the first exemplary embodiment, aperiod during which the light beam is deflected by one mirror surface ofthe polygon mirror 37 is defined as one cycle. Therefore, in the firstexemplary embodiment, it takes six cycles for the polygon mirror 37 tomake one rotation.

The BD light-receiving sensor 51 (one example of a sensor) (hereinafter,referred to as “BD sensor”) is placed at a position at which the lightbeam deflected by the polygon mirror 37 passes before the light beampasses through the photosensitive drum 30. It is noted that the BDsensor 51 may be optionally placed at a position at which the light beampasses after the light beam passes through the photosensitive drum 30.The BD sensor 51 receives the light beam caused to emit light from theLD 47, and outputs a BD (Beam Detect) signal (one example of a detectionsignal) to the CPU 60. The BD signal is sometimes called a horizontalsynchronization signal.

An APC light-receiving sensor 52 (one example of a sensor) (hereinafter,referred to as “APC sensor”) is placed near the LD 47. Once the LD 47emits a light beam, the APC sensor 52 receives the light beam constantlyand outputs to the LD driving circuit 48 a sensor output (one example ofa detection signal) that is according to the strength of the receivedlight beam.

In the photosensitive drum 30, a region 53 is an image forming region 53in which an electrostatic latent image is formed. In the first exemplaryembodiment, the electrostatic latent image is not formed on the entiretop surface of the photosensitive drum 30, but formed in the imageforming region 53 that is inside the entire top surface excluding theboth ends in an axial direction of the photosensitive drum 30.

(4) Rotation Control of Polygon Mirror

The CPU 60 detects the rotation speed of the polygon mirror 37 from atime interval during which the BD signal is output, and performsfeedback control on the polygon motor so that the rotation speed of thepolygon mirror 37 maintains a predetermined speed.

In the first exemplary embodiment, not only while the rotation speed ofthe photosensitive drum 30 is equal to or more than the predeterminedspeed (while it is determined that the rotation speed is high) but alsowhile the rotation speed of the photosensitive drum 30 is less than thepredetermined speed (while it is determined that the rotation speed islow), the CPU 60 continuously maintains the rotation speed of thepolygon mirror 37 at the predetermined speed based on the BD signal.While the rotation speed of the photosensitive drum 30 is less than thepredetermined speed, the electrostatic latent image is not formed in theimage forming region 53. Thus, during that period, the rotation of thepolygon mirror 37 may be stopped. However, if the rotation of thepolygon mirror 37 is stopped, it requires a certain time until therotation speed becomes stable when the rotation of the polygon mirror 37is resumed. Therefore, in the first exemplary embodiment, even while therotation speed of the photosensitive drum 30 is less than thepredetermined speed, the rotation speed of the polygon mirror 37 iscontinuously maintained at the predetermined speed.

In this case, whether the rotation speed of the photosensitive drum 30is equal to or more than the predetermined speed can be determined asfollows: For example, the CPU 60 calculates the rotation speed of thephotosensitive drum 30 by' using a pulse signal output to the conveyingmotor M, a timer (not shown), etc. In this case, when the rotation speedof the photosensitive drum 30 is equal to or more than the predeterminedspeed, the CPU 60 determines that the rotation speed is high, and whenthe rotation speed is less than the predetermined speed, the CPU 60determines that the rotation speed is low. Alternately, the number ofrotations of an encoder, which is arranged in the photosensitive drum30, may be detected by a photoelectric sensor, and based on thedetection result and a certain time counted by a timer, the rotationspeed may be detected.

(5) Auto Power Control (APC)

The LD driving circuit 48 automatically adjusts the intensity of thelight beam (LD light-emission intensity) caused to emit light from theLD 47 by adjusting a value of a current supplied to the laser diodebased on the sensor output from the APC light-receiving sensor 52. Thisautomatic adjustment is called APC.

In the first exemplary embodiment, not only while the rotation speed ofthe photosensitive drum 30 is equal to or more than the predeterminedspeed but also while the rotation speed of the photosensitive drum 30 isless than the predetermined speed, the intensity of the light beam isadjusted by the APC. The reason for this is that when the APC isstopped, it requires a certain time until the intensity of the lightbeam becomes stable when the APC is resumed.

(6) Rotation Control of Photosensitive Drum

FIG. 4 is a timing chart for explaining the rotation control, by the CPU60, of the photosensitive drum 30. Herein, rotation control in a periodduring which the formation of an image on the top surface of the printsheet 15 is finished until the formation of the image on the bottomsurface is enabled after the print sheet 15 is reversed will beexplained as an example.

A period from a time point T1 until a time point T2 is a standby statein which the image forming apparatus 1 stands by after the image formingapparatus 1 has finished forming the image on the top surface of theprint sheet 15. In the standby state, the photosensitive drum 30 makes aconstant speed rotation at a constant speed.

When the operation for reversing the print sheet 15 is started at thetime point T2, the CPU 60 decreases the rotation speed of thephotosensitive drum 30 by gradually decreasing the rotation speed of theconveying motor M. In the illustrated example, at a time point T3, therotation speed of the photosensitive drum 30 reaches the predeterminedspeed, and at a time point T4, the speed of the photosensitive drum 30reaches 0 [rpm].

When the speed of the photosensitive drum 30 reaches 0 [rpm] at the timepoint T4, the CPU 60 stands by until a time point T5. At the time pointT5, the CPU 60 starts reversing the rotation of the conveying motor M.The driving force for reversing the rotation of the conveying motor M isnot transmitted to the photosensitive drum 30, and thus, thephotosensitive drum 30 remains stopped.

When the rotation speed of the conveying motor M reaches a constantrotation speed at a time point T6, the CPU 60 causes the conveying motorM to rotate while maintaining the constant rotation speed until a timepoint T7 at which reversing the print sheet 15 is completed. When thetime point T7 is reached, the CPU 60 gradually decreases the rotationspeed of the conveying motor M.

When the rotation speed of the conveying motor M reaches 0 [rpm] at atime point T8, the CPU 60 stands by until a time point T9. At the timepoint T9, the CPU 60 again starts the positive rotation of the conveyingmotor M. Thereby, the rotation of the photosensitive drum 30 is resumed.

When the rotation speed of the conveying motor M reaches a constantspeed at a time point T10, the photosensitive drum 30 makes a constantspeed rotation at the constant speed described above. This enables theformation of the image on the bottom surface. This is followed by theformation of the image on the bottom surface of the print sheet 15.

(7) Light-Emission Control of Laser Diode

FIG. 5 is a timing chart showing timings at which various types ofsignals and currents are output. For the sake of understanding, the“rotation speed (rpm) of the photosensitive drum” is shown together inthe figure.

(7-1) Video Signal

The Video signal is output when the electrostatic latent image is formedin the image forming region 53 and when the light beam is emitted inorder to obtain the BD signal. Moreover, as described above, when the LD47 emits the light beam, the APC sensor 52 receives the light beamconstantly, and thus, it can be said that the Video signal output inorder to form the electrostatic latent image in the image forming region53 and the Video signal output in order to obtain the BD signal are bothsignals used for obtaining a sensor output from the APC light-receivingsensor 52.

It is when the rotation speed of the photosensitive drum 30 is equal toor more than the predetermined speed that the Video signal used forforming the electrostatic latent image in the image forming region 53 isoutput. However, FIG. 5 shows a period during which the formation of theimage on the top surface of the print sheet 15 is finished until theformation of the image on the bottom surface thereof is started. Thus,the Video signal used for forming the electrostatic latent image in theimage forming region 53 is not shown.

When the electrostatic latent image is formed in the image formingregion 53, the CPU 60 outputs to the LD driving circuit 48 the Videosignal used for forming the electrostatic latent image in the imageforming region 53 at a timing that is based on the BD signal. The LDdriving circuit 48 supplies the LD 47 with the switching current whenthe Video signal is output. As a result, a high-intensity light beam isemitted from the LD 47. With this light beam, the image forming region53 is exposed, and as a result, one line of the electrostatic latentimage according to the Video signal is formed in the image formingregion 53.

It is when the deflecting direction of the light beam is oriented atleast toward the BD sensor 51 that the Video signal used for obtainingthe BD signal is output regardless of whether the rotation speed of thephotosensitive drum 30 is equal to or more than the predetermined speed.The Video signals shown in FIG. 5 are all Video signals used forobtaining the BD signal. When the deflecting direction of the light beamis oriented at least toward the BD sensor 51, the CPU 60 outputs to theLD driving circuit 48 the Video signal used for obtaining the BD signal.

In this case, “when being oriented at least toward the BD sensor 51” mayoptionally include a case where being oriented toward other than the“image forming region 53” if when being oriented toward the BD sensor 51is included. After all, when being oriented toward the “image formingregion 53” is not included in “when being oriented at least toward theBD sensor 51.”

Whether the deflecting direction of the light beam is oriented towardthe BD sensor 51 can be determined by a value on a counter for countinga clock signal, for example. The CPU 60 regards a period during whichthe value on the counter is within a specific range (for example, 100 to200) as a period during which the deflecting direction of the light beamis oriented toward the BD sensor 51, and outputs the Video signal.

The reason why the Video signal used for obtaining the BD signal isoutput even when the rotation speed of the photosensitive drum 30 isless than the predetermined speed is, as described above, to performrotation control of the polygon mirror 37 and the APC even when therotation speed of the photosensitive drum 30 is less than thepredetermined speed.

The LD driving circuit 48 supplies the LD 47 with the switching currentwhen the Video signal is output. As a result, a high-intensity lightbeam is emitted from the LD 47. The light beam is received by the BDsensor 51, and the BD signal is output to the CPU 60.

A “current value of current supplied to the LD” shown in FIG. 5 is acombined current of the switching current and the bias current in aperiod during which the bias current described below is supplied, and isonly the switching current in a period during which the bias current isnot supplied. A portion shaded in the “current value of current suppliedto the LD” indicates the bias current.

(7-2) Bias Signal

The bias signal is output when the bias current is supplied.

It is when the rotation speed of the photosensitive drum 30 is equal toor more than the predetermined speed that the bias signal is output. TheCPU 60 calculates the rotation speed of the photosensitive drum 30 froma pulse signal output to the conveying motor M, a timer, etc., forexample. The CPU 60 determines that the rotation speed is high when therotation speed of the photosensitive drum 30 is equal to or more thanthe predetermined speed, and in this case, outputs the bias signal tothe LD driving circuit 48 constantly regardless of whether thedeflecting direction of the light beam is oriented toward thephotosensitive drum 30 or toward the BD sensor 51. The reason for thisis to reduce a decrease in responsiveness of the LD 47. Once supplyingthe current is stopped, the LD 47 requires a time until the LD 47 emitsthe light beam even when the LD 47 is thereafter supplied with thecurrent. When the bias current is supplied, it becomes possible toreduce a decrease in responsiveness of the LD 47 caused when theswitching current is supplied.

The LD 47 emits light by the bias current, and the light emitted by thebias current is low-intensity light that is obtained when thephotosensitive drum 30 is exposed with an intensity of less than apredetermined exposure amount. Thus, the photosensitive drum 30 is notgreatly affected. Moreover, it is when the rotation speed of thephotosensitive drum 30 is equal to or more than the predetermined speedthat the bias current is supplied. Therefore, the same location of thephotosensitive drum 30 will not be intensively irradiated with thelight.

The purpose of the APC is to adjust the intensity of the light beamemitted by the Video signal, and for this reason, an adjustment in whichthe intensity of the light emitted by the bias signal when the Videosignal is not output is equal to the “intensity of the light beamemitted by the Video signal” is not performed.

The CPU 60 stops outputting the bias signal when the rotation speed ofthe photosensitive drum 30 is less than the predetermined speed. Thereason for this is that the same location of the photosensitive drum 30may be intensively irradiated with the light emitted by the bias currentwhen the rotation speed of the photosensitive drum 30 is less than thepredetermined speed. If the same location is intensively irradiated withlight even if the light is low in intensity, the deterioration of thephotosensitive drum 30 may be accelerated. Therefore, when the rotationspeed of the photosensitive drum 30 is less than the predeterminedspeed, the CPU 60 stops the bias signal.

(7-3) Light Emission of the Light Beam to the Image Forming Region whenthe Rotation Speed of the Photosensitive Drum is Less than thePredetermined Speed

It is when the rotation speed of the photosensitive drum 30 is equal toor more than the predetermined speed that the Video signal used forforming the electrostatic latent image in the image forming region 53 isoutput. Thus, when the rotation speed of the photosensitive drum 30 isless than the predetermined speed, the Video signal used for forming theelectrostatic latent image is not output even when the deflectingdirection of the light beam is oriented toward the image forming region53.

The Video signal used for obtaining the BD signal is output regardlessof whether the rotation speed of the photosensitive drum 30 is equal toor more than the predetermined speed, but it is when the deflectingdirection of the light beam is oriented at least toward the BD sensor 51(which does not include when being oriented toward the “image formingregion 53”) that the Video signal used for obtaining the BD signal isoutput. Therefore, the Video signal is not output when the deflectingdirection of the light beam is oriented toward the image forming region53.

Moreover, it is when the rotation speed of the photosensitive drum 30 isequal to or more than the predetermined speed that the bias signal isoutput. Thus, when the rotation speed of the photosensitive drum 30 isless than the predetermined speed, the bias signal is not output.

That is, if the rotation speed of the photosensitive drum 30 is lessthan the predetermined speed, neither signal is output when thedeflecting direction of the light beam is oriented toward the imageforming region 53 of the photosensitive drum 30. That is, when therotation speed of the photosensitive drum 30 is less than thepredetermined speed, the light beam will not be emitted toward the imageforming region 53 of the photosensitive drum 30.

It is noted that when the rotation speed of the photosensitive drum 30is less than the predetermined speed, the light beam should not beemitted when the deflecting direction is oriented toward at least theimage forming region 53, and the light beam may be optionally emittedwhen the deflecting direction is oriented toward the photosensitive drum30 other than the image forming region 53.

(8) Adjustment of Switching Current

When the rotation speed of the photosensitive drum 30 is equal to ormore than the predetermined speed, the bias signal is output, and thus,the Video signal used for forming the electrostatic latent image in theimage forming region 53 or the Video signal used for obtaining the BDsignal is output, the combined current of the bias current and theswitching current is supplied to the LD 47.

On the other hand, when the rotation speed of the photosensitive drum 30is less than the predetermined speed, the bias signal is not output.Thus, when the Video signal used for obtaining the BD signal is output,only the switching current is supplied.

That is, the value of the current, which is supplied to the LD 47 whenthe deflecting direction of the light beam is oriented toward the BDsensor 51, differs depending on whether the rotation speed of thephotosensitive drum 30 is equal to or more than the predetermined speedor less than the predetermined speed. Immediately after the rotationspeed has fallen below the predetermined speed, only a small currentreduced by as much as the bias current is supplied.

When the current to be supplied is small, the intensity of the lightbeam becomes small corresponding thereto, and thus, the value of theswitching current is adjusted by the APC. As a result, the value of theswitching current supplied to the LD 47 when the rotation speed is lessthan the predetermined speed is adjusted to the substantially the samevalue as the combined current obtained by combining the bias current andthe switching current and supplied to the LD 47 when the rotation speedof the photosensitive drum 30 is equal to or more than the predeterminedspeed.

(9) State of LD Driving Circuit

The LD driving circuit 48 is maintained in an operable state capable ofsupplying a current to the LD 47 even when the rotation speed of thephotosensitive drum 30 is less than the predetermined speed.

When the rotation speed of the photosensitive drum 30 is less than thepredetermined speed, the light beam is not emitted if the deflectingdirection of at least the light beam is oriented toward the imageforming region 53. Thus, during that period, it is conceivable todecrease the power consumption by bringing the LD driving circuit 48 ina standby state where no operation is enabled. However, the LD drivingcircuit 48 requires a certain amount of time until restore from thestandby state to the operable state. This may delay supplying thecurrent, possibly resulting in delaying the light emission of the lightbeam.

Even when the rotation speed of the photosensitive drum 30 is less thanthe predetermined speed, such a delay in the light emission of the lightbeam can be decreased when the LD driving circuit 48 is maintained inthe operable state.

According to the image forming apparatus 1 based on the first exemplaryembodiment of the present invention as described above, even when therotation speed of the photosensitive drum 30 is less than thepredetermined speed in the period during which the image formingapparatus 1 should be controlled, for example, when the rotation of thephotosensitive drum 30 is stopped as a result of use of the power of theconveying motor M when the conveying mechanism 13 reverses the printsheet 15, as long as the deflecting direction of the light beam isoriented toward the sensor, the light beam is emitted. Therefore, evenwhen the rotation speed of the photosensitive drum 30 is less than thepredetermined speed, it is possible to control the image formingapparatus 1 based on the light beam as in the rotation control of thepolygon mirror and the APC.

Moreover, according to the image forming apparatus 1, when the rotationspeed of the photosensitive drum 30 is less than the predeterminedspeed, if the deflecting direction of the light beam is oriented towardthe image forming region 53, the light beam is not emitted. Thus, it ispossible to inhibit the deterioration of the photosensitive drum 30, inparticular, the deterioration of the image forming region 53, causedwhen the same location of the image forming region 53 is intensivelyirradiated with the light beam.

Therefore, according to the image forming apparatus 1, when the rotationspeed of the photosensitive drum 30 is low, it is possible to controlthe image forming apparatus 1 based on the light beam while inhibitingthe deterioration of the photosensitive drum 30.

Moreover, according to the image forming apparatus 1, when thedeflecting direction of the light beam is oriented toward the BD sensor51, the high-intensity light beam is emitted. As a result, the accuracyfor detecting the light beam is improved.

Moreover, according to the image forming apparatus 1, when the rotationspeed of the photosensitive drum 30 is equal to or more than thepredetermined speed, the bias current is constantly supplied to the LD47, and thereby, it becomes possible to improve the responsiveness ofthe LD 47. Moreover, this bias current is a low current with which toemit low-intensity light that is obtained when the photosensitive drum30 is exposed with an intensity of less than a predetermined exposureamount. Thus, it is possible to inhibit the deterioration of thephotosensitive drum 30, caused when the bias current is constantlysupplied.

Moreover, according to the image forming apparatus 1, the LD drivingcircuit 48 is maintained in the operable state even when the rotationspeed of the photosensitive drum 30 is less than the predeterminedspeed. Thus, it is possible to reduce a decrease in responsiveness ofthe LD driving circuit 48.

(Second Exemplary Embodiment)

Subsequently, a second exemplary embodiment of the present inventionwill be described with reference to FIG. 6.

In the second exemplary embodiment, when the rotation speed of thephotosensitive drum 30 is less than the predetermined speed, a timing atwhich to output the Video signal used for obtaining the BD signal isfaster than a timing at which to output the Video signal used forobtaining the BD signal when the rotation speed of the photosensitivedrum 30 is equal to or more than the predetermined speed.

In other words, when the rotation speed of the photosensitive drum 30 isless than the predetermined speed, at the time of causing the LD 47 toemit light in order to obtain the BD signal, a timing at which to startsupplying the switching current is faster than a timing at which tostart supplying the switching current when the rotation speed of thephotosensitive drum 30 is equal to or more than the predetermined speed.

FIG. 6 is a timing chart according to the second exemplary embodiment.Also in the second exemplary embodiment, when the rotation speed of thephotosensitive drum 30 is less than the predetermined speed, the biascurrent is not supplied. A region indicated by an oblique line in aVideo signal shown in the FIG. 6 indicates a zone in which the Videosignal is output at an advanced timing.

An adjustment signal shown in FIG. 6 is a signal used for setting atarget intensity when the intensity of the light beam (LD light-emissionintensity) is automatically adjusted by the APC, and is output from theCPU 60 to the LD driving circuit 48. The LD driving circuit 48 executesthe APC so that the intensity of the light beam reaches this targetintensity. In the second exemplary embodiment, the CPU 60 outputs to theLD driving circuit 48 an adjustment signal that makes the value of theswitching current output when the rotation speed of the photosensitivedrum 30 is less than the predetermined speed large so as to becomesubstantially the same value as the combined current obtained bycombining the bias current and the switching current of the switchingcurrent output when the rotation speed of the photosensitive drum 30 isequal to or more than the predetermined speed. According thereto, in thesecond exemplary embodiment, even when the rotation speed of thephotosensitive drum 30 is either equal to or more than the predeterminedspeed or less than the predetermined speed, the LD light-emissionintensity is set to constant.

Although in FIG. 5 in the first exemplary embodiment, the LDlight-emission intensity and the adjustment signal are not shown, thewaveforms of the LD light-emission intensity and the adjustment signalin the first exemplary embodiment are the same as those in the secondexemplary embodiment.

When the rotation speed of the photosensitive drum 30 is less than thepredetermined speed, if the timing at which to output the Video signalused for obtaining the BD signal is advanced, the timing at which tostart causing the LD 47 to emit light is advanced. Thus, it becomespossible to reduce the decrease in responsiveness of the LD 47.

In other respects, the second exemplary embodiment is substantiallyidentical to the first exemplary embodiment.

(Third Exemplary Embodiment)

Subsequently, a third exemplary embodiment of the present invention willbe described with reference to FIG. 7.

In the first and second exemplary embodiments as described above, whenthe rotation speed of the photosensitive drum 30 is less than thepredetermined speed, the switching current becomes large by the APC.However, when the switching current is made large, it is probable in acertain design that a current exceeding a rating of the switchingcurrent circuit within the LD driving circuit 48 passes.

In the third exemplary embodiment, when the rotation speed of thephotosensitive drum 30 is less than the predetermined speed, if thedeflecting direction of the light beam is oriented toward the BD sensor51, a switching current, which is substantially the same value as thatof the switching current supplied when the rotation speed of thephotosensitive drum 30 is equal to or more than the predetermined speed,is supplied.

FIG. 7 is a timing chart according to the third exemplary embodiment. Inthe third exemplary embodiment, the LD light-emission intensity obtainedwhen the rotation speed of the photosensitive drum 30 is less than thepredetermined speed is made smaller than that obtained when the rotationspeed of the photosensitive drum 30 is equal to or more than thepredetermined speed. Specifically, the CPU 60 outputs to the LD drivingcircuit 48 an adjustment signal that would substantially equal the valueof a current supplied to the LD 47 obtained when the rotation speed ofthe photosensitive drum 30 is less than the predetermined speed, i.e.,the value of the switching current, to the value of the switchingcurrent obtained when the rotation speed of the photosensitive drum 30is equal to or more than the predetermined speed. Thereby, it becomespossible to ensure that the switching current is not made larger than acase where the rotation speed of the photosensitive drum 30 is equal toor more than the predetermined speed, and possible to ensure that thecurrent passing through the switching current circuit does not exceedthe rating of the switching current circuit.

It is noted that when the rotation speed of the photosensitive drum 30is less than the predetermined speed, merely obtaining the BD signal issufficient. Thus, a target value of the switching current to becontrolled by the APC may be, for example, approximately half the valueof the switching current supplied when the rotation speed of thephotosensitive drum 30 is equal to or more than the predetermined speed.

In other respects, the third exemplary embodiment is substantiallyidentical to the first exemplary embodiment.

(Fourth Exemplary Embodiment)

Subsequently, a fourth exemplary embodiment of the present inventionwill be described with reference to FIG. 8.

In the fourth exemplary embodiment, even when the rotation speed of thephotosensitive drum 30 is less than the predetermined speed, if thedeflecting direction of the light beam is oriented toward the BD sensor51, the bias current is supplied. The reason for this is to make itdifficult to easily cause the following problem resulting from notsupplying the bias current.

When the bias current is not supplied, the switching current is greatlyfluctuated because the switching current is fluctuated by the APC by asmuch as the bias current. Thus, there is a possibility that the controlby the LD driving circuit 48 becomes unstable.

There is a slight time lag in the adjustment by the APC, and thus, whenthe bias current is not supplied, there is a possibility that thecurrent supplied to the LD 47 becomes small for an instant whensupplying the bias current is stopped. On the other hand, there is apossibility that when supplying the bias current is started, the currentsupplied to the LD 47 becomes large for an instant.

FIG. 8 is a timing chart according to the fourth exemplary embodiment.If the bias current is to be supplied when the deflecting direction ofthe light beam is oriented toward the BD sensor 51 even in the casewhere the rotation speed of the photosensitive drum 30 is less than thepredetermined speed, it becomes possible to make it difficult to easilycause the above-described problem that results from not supplying thebias current.

A period during which the bias current is supplied when the rotationspeed of the photosensitive drum 30 is less than the predetermined speedmay optionally include a period during which the deflecting direction ofthe light beam is not oriented toward the BD sensor 51 as long as aperiod during which at least the deflecting direction of the light beamis oriented toward the BD sensor 51 is included. However, in this case,a period during which at least the deflecting direction of the lightbeam is oriented toward the image forming region 53 is excluded.

In other respects, the fourth exemplary embodiment is substantiallyidentical to the first exemplary embodiment.

(Modified Exemplary Embodiments)

The present invention is not limited to the above-described exemplaryembodiments. For example, an embodiment as set forth below may also beincluded in the technical scope of the present invention.

In the first to fourth embodiments, as an example, the rotation controlof the polygon mirror and the APC have been described as control that isbased on the detection signal output from the sensor. However, thecontrol that is based on the detection signal output from the sensor isnot limited thereto. Another control may be accepted as long as thecontrol is control that is performed based on the detection signal evenwhen the rotation speed of the photosensitive drum 30 is less than thepredetermined speed.

In the first to fourth embodiments, as an example, a so-calledmonochrome printer for printing by using single-color toner has beendescribed as the image forming apparatus 1. However, the image formingapparatus may be optionally a color printer using two or more colors forthe toner.

In the first and fourth embodiments, as the image forming apparatus, theimage forming apparatus 1 has been described as an example. However, amultifunction device including a scanner function, a printer function, acopy function, a facsimile function, etc., may be optionally applied tothe present invention.

While the present invention has been shown and described with referenceto certain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the scope of the invention as defined bythe appended claims.

What is claimed is:
 1. An image forming apparatus comprising: a lightsource configured to emit a light beam; a light-source driving unitconfigured to drive the light source; a photosensitive member thatcomprises an image forming region; a determining unit configured todetermine a level of a rotation speed of the photosensitive member; ascanner unit configured to expose the photosensitive member bydeflecting the light beam emitted from the light source by a deflectingunit; a sensor configured to receive the light beam and output adetection signal; and a control unit configured to control the imageforming apparatus based on the detection signal output from the sensor,wherein when the determining unit determines that the rotation speed islow, the light-source driving unit is configured: to drive the lightsource to emit the light beam when a deflecting direction of the lightbeam is oriented toward the sensor; and not to drive the light source toemit the light beam when the deflecting direction of the light beam isoriented toward the image forming region, wherein an intensity of thelight beam emitted from the light source is configured to vary accordingto a current supplied to the light source, wherein the light-sourcedriving unit is configured to supply: a first current configured todrive the light source to emit a light beam having a first intensity forexposing the photosensitive member with an intensity of greater than orequal to a predetermined exposure amount; and a second currentconfigured to drive the light source to emit a light having a secondintensity for exposing the photosensitive member with an intensity ofless than the predetermined exposure amount, wherein when the deflectingdirection of the light beam is oriented toward the sensor, thelight-source driving unit is configured to supply the first current tothe light source, wherein when the determining unit determines that therotation speed of the photosensitive member is high, the light-sourcedriving unit is configured to constantly supply the second current as abias current to the light-source driving unit, and wherein when thedetermining unit determines that the rotation speed of thephotosensitive member is low, the light-source driving unit isconfigured to stop supplying the bias current to the light source and tomake a timing at which the light source is supplied with the firstcurrent faster than a timing at which the first current is supplied whenthe rotation speed of the photosensitive member is high.
 2. The imageforming apparatus according to claim 1, wherein the light-source drivingunit is kept in an operable state even when the rotation speed of thephotosensitive member is decreased.
 3. The image forming apparatusaccording to claim 1, further comprising: a speed detecting unitconfigured to detect the rotation speed of the photosensitive member,wherein the determining unit is configured to determined the level ofspeed by comparing a speed detected by the speed detecting unit and apredetermined speed.
 4. The image forming apparatus according to claim1, further comprising: a photosensitive-member driving unit configuredto rotationally drive the photosensitive member; and a conveying unitconfigured to convey a recorded medium one at a time, and, after animage is formed on a first surface of the recorded medium, to reverseand convey the recorded medium by using power of thephotosensitive-member driving unit in order to form an image on a secondsurface of the recorded medium, wherein when the conveying unit reversesthe recorded medium, the power of the photosensitive-member driving unitis applied to the conveying unit so as to stop a rotation of thephotosensitive member.
 5. An image forming apparatus comprising: a lightsource configured to emit a light beam; a light-source driving unitconfigured to drive the light source; a photosensitive member thatcomprises an image forming region; a determining unit configured todetermine a level of a rotation speed of the photosensitive member; ascanner unit configured to expose the photosensitive member bydeflecting the light beam emitted from the light source by a deflectingunit; a sensor configured to receive the light beam and output adetection signal; and a control unit configured to control the imageforming apparatus based on the detection signal output from the sensor,wherein when the determining unit determines that the rotation speed islow, the light-source driving unit is configured: to drive the lightsource to emit the light beam when a deflecting direction of the lightbeam is oriented toward the sensor; and not to drive the light source toemit the light beam when the deflecting direction of the light beam isoriented toward the image forming region, wherein an intensity of thelight beam emitted from the light source is configured to vary accordingto a current supplied to the light source, wherein the light-sourcedriving unit is configured to supply: a first current configured todrive the light source to emit a light beam having a first intensity forexposing the photosensitive member with an intensity of greater than orequal to a predetermined exposure amount; and a second currentconfigured to drive the light source to emit a light having a secondintensity for exposing the photosensitive member with an intensity ofless than the predetermined exposure amount, wherein when the deflectingdirection of the light beam is oriented toward the sensor, thelight-source driving unit is configured to supply the first current tothe light source, wherein when the determining unit determines that therotation speed of the photosensitive member is high, the light-sourcedriving unit is configured to constantly supply the second current as abias current to the light-source driving unit, and wherein when thedetermining unit determines that the rotation speed of thephotosensitive member is low, the light-source driving unit isconfigured to stop supplying the bias current to the light source and toadjust a value of the first current supplied to the light source tosubstantially the same value as the first current supplied to the lightsource when the rotation speed of the photosensitive member isdetermined to be high.
 6. The image forming apparatus according to claim5, wherein the light-source driving unit is kept in an operable stateeven when the rotation speed of the photosensitive member is decreased.7. The image forming apparatus according to claim 5, further comprising:a speed detecting unit configured to detect the rotation speed of thephotosensitive member, wherein the determining unit is configured todetermine the level of speed by comparing a speed detected by the speeddetecting unit and a predetermined speed.
 8. The image forming apparatusaccording to claim 5, further comprising: a photosensitive-memberdriving unit configured to rotationally drive the photosensitive member;and a conveying unit configured to convey a recorded medium one at atime, and, after an image is formed on a first surface of the recordedmedium, to reverse and convey the recorded medium by using power of thephotosensitive-member driving unit in order to form an image on a secondsurface of the recorded medium, wherein when the conveying unit reversesthe recorded medium, the power of the photosensitive-member driving unitis applied to the conveying unit so as to stop a rotation of thephotosensitive member.
 9. An image forming apparatus comprising: a lightsource configured to emit a light beam; a light-source driving unitconfigured to drive the light source; a photosensitive member thatcomprises an image forming region; a determining unit configured todetermine a level of a rotation speed of the photosensitive member; ascanner unit configured to expose the photosensitive member bydeflecting the light beam emitted from the light source by a deflectingunit; a sensor configured to receive the light beam and output adetection signal; and a control unit configured to control the imageforming apparatus based on the detection signal output from the sensor,wherein when the determining unit determines that the rotation speed islow, the light-source driving unit is configured: to drive the lightsource to emit the light beam when a deflecting direction of the lightbeam is oriented toward the sensor; and not to drive the light source toemit the light beam when the deflecting direction of the light beam isoriented toward the image forming region, wherein an intensity of thelight beam emitted from the light source is configured to vary accordingto a current supplied to the light source, wherein the light-sourcedriving unit is configured to supply: a first current configured todrive the light source to emit a light beam having a first intensity forexposing the photosensitive member with an intensity of greater than orequal to a predetermined exposure amount; and a second currentconfigured to drive the light source to emit a light having a secondintensity for exposing the photosensitive member with an intensity ofless than the predetermined exposure amount, wherein when the deflectingdirection of the light beam is oriented toward the sensor, thelight-source driving unit is configured to supply the first current tothe light source, wherein when the determining unit determines that therotation speed of the photosensitive member is high, the light-sourcedriving unit is configured to constantly supply the second current as abias current to the light-source driving unit, and wherein when thedetermining unit determines that the rotation speed of thephotosensitive member is low, the light-source driving unit isconfigured to stop supplying the bias current to the light source and toadjust a value of the first current supplied to the light source to avalue less than that of the first current supplied to the light sourcewhen the rotation speed of the photosensitive member is determined to behigh.
 10. The image forming apparatus according to claim 9, wherein thelight-source driving unit is kept in an operable state even when therotation speed of the photosensitive member is decreased.
 11. The imageforming apparatus according to claim 9, further comprising: a speeddetecting unit configured to detect the rotation speed of thephotosensitive member, wherein the determining unit is configured todetermine the level of speed by comparing a speed detected by the speeddetecting unit and a predetermined speed.
 12. The image formingapparatus according to claim 9, further comprising: aphotosensitive-member driving unit configured to rotationally drive thephotosensitive member; and a conveying unit configured to convey arecorded medium one at a time, and, after an image is formed on a firstsurface of the recorded medium, to reverse and convey the recordedmedium by using power of the photosensitive-member driving unit in orderto form an image on a second surface of the recorded medium, whereinwhen the conveying unit reverses the recorded medium, the power of thephotosensitive-member driving unit is applied to the conveying unit soas to stop a rotation of the photosensitive member.
 13. An image formingapparatus comprising: a light source configured to emit a light beam; alight-source driving unit configured to drive the light source; aphotosensitive member that comprises an image forming region; adetermining unit configured to determine a level of a rotation speed ofthe photosensitive member; a scanner unit configured to expose thephotosensitive member by deflecting the light beam emitted from thelight source by a deflecting unit; a sensor configured to receive thelight beam and output a detection signal; and a control unit configuredto control the image forming apparatus based on the detection signaloutput from the sensor, wherein when the determining unit determinesthat the rotation speed is low, the light-source driving unit isconfigured: to drive the light source to emit the light beam when adeflecting direction of the light beam is oriented toward the sensor;and not to drive the light source to emit the light beam when thedeflecting direction of the light beam is oriented toward the imageforming region, wherein an intensity of the light beam emitted from thelight source is configured to vary according to a current supplied tothe light source, wherein the light-source driving unit is configured tosupply: a first current configured to drive the light source to emit alight beam having a first intensity for exposing the photosensitivemember with an intensity of greater than or equal to a predeterminedexposure amount; and a second current configured to drive the lightsource to emit a light having a second intensity for exposing thephotosensitive member with an intensity of less than the predeterminedexposure amount, wherein when the deflecting direction of the light beamis oriented toward the sensor, the light-source driving unit isconfigured to supply the first current to the light source, wherein whenthe determining unit determines that the rotation speed of thephotosensitive member is high, the light-source driving unit isconfigured to constantly supply the second current as a bias current tothe light-source driving unit, and wherein when the determining unitdetermines that the rotation speed of the photosensitive member is low,the light-source driving unit is configured: not to supply the biascurrent when the deflecting direction of the light beam is orientedtoward the image forming region; and to supply the bias current when thedeflecting direction of the light beam is oriented toward the sensor.14. The image forming apparatus according to claim 13, wherein thelight-source driving unit is kept in an operable state even when therotation speed of the photosensitive member is decreased.
 15. The imageforming apparatus according to claim 13, further comprising: a speeddetecting unit configured to detect the rotation speed of thephotosensitive member, wherein the determining unit is configured todetermine the level of speed by comparing a speed detected by the speeddetecting unit and a predetermined speed.
 16. The image formingapparatus according to claim 13, further comprising: aphotosensitive-member driving unit configured to rotationally drive thephotosensitive member; and a conveying unit configured to convey arecorded medium one at a time, and, after an image is formed on a firstsurface of the recorded medium, to reverse and convey the recordedmedium by using power of the photosensitive-member driving unit in orderto form an image on a second surface of the recorded medium, whereinwhen the conveying unit reverses the recorded medium, the power of thephotosensitive-member driving unit is applied to the conveying unit soas to stop a rotation of the photosensitive member.